1. Field of the Invention
The present invention generally relates to catalysts and processes for converting a light hydrocarbon (e.g., natural gas) and oxygen to a product comprising a mixture of carbon monoxide and hydrogen (“synthesis gas” or “syngas”). More particularly, the invention relates to such processes and catalysts employing Ni—MgO containing catalysts.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane and other light hydrocarbons (i.e., C1-C5 hydrocarbons) as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or “syngas”). In a second step, the syngas is converted to hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.CH4+H2O⇄CO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. For many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations. Methane residence times in steam reforming are on the order of 0.5-1 second, whereas for heterogeneously catalyzed partial oxidation, the residence time is on the order of a few milliseconds. For the same production capacity, syngas facilities for the partial oxidation of methane can be far smaller, and less expensive, than facilities based on steam reforming. A recent report (M. Fichtner et al., Ind. Eng. Chem. Res. (2001) 40:3475-3483) states that for efficient syngas production, the use of elevated operation pressures of about 2.5 MPa is required. Those authors describe a partial oxidation process in which the exothermic complete oxidation of methane is coupled with the subsequent endothermic reforming reactions (water and CO2 decomposition). This type of process can also be referred to as autothermal reforming or ATR, especially when steam is co-fed with the methane. Certain microstructured rhodium honeycomb catalysts are employed which have the advantage of a smaller pressure drop than beds or porous solids (foams) and which resist the reaction heat of the total oxidation reaction taking place at the catalyst inlet.
The catalytic partial oxidation (“CPOX”) or direct partial oxidation of hydrocarbons (e.g., natural gas or methane) to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+1/2O2→CO+2H2  (2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol or to fuels. The CPOX reaction is exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes that is possible in a conventional steam reforming process.
While its use is currently limited as an industrial process, the direct partial oxidation or CPOX of methane has recently attracted much attention due to its inherent advantages, such as the fact that due to the significant heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction, in contrast to steam reforming processes. An attempt to overcome some of the disadvantages and costs typical of steam reforming by production of synthesis gas via the catalytic partial oxidation of methane is described in European Patent No. 303,438. According to that method, certain high surface area monoliths coated with metals or metal oxides that are active as oxidation catalysts, e.g., Pd, Pt, Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La, and mixtures thereof, are employed as catalysts. Other suggested coating metals are noble metals and metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic table of the elements.
U.S. Pat. No. 5,149,464 describes a method for selectively converting methane to syngas at 650-950° C. by contacting a methane/oxygen mixture with a solid catalyst which is a d-block transition metal on a refractory support, an oxide of a d-block transition metal, or a compound of the formula MxM′yOz wherein M′ is a d-block transition metal and M is Mg, B, Al, Ga, Si, Ti, Zr, Hf or a lanthanide. U.S. Pat. No. 5,500,149 describes the combination of dry reforming and partial oxidation of methane, in the presence of added CO2 to enhance the selectivity and degree of conversion to synthesis gas. U.S. Pat. No. 5,431,855 demonstrates the catalytic conversion of mixtures of CO2, O2 and CH4 to synthesis gas over selected alumina supported transition metal catalysts. Maximum CO yield reported was 89% at a gas hourly space velocity (GHSV) of 1.5×104 hr−1, temperature of 1,050° K and pressure of 100 kPa. The addition of CO2 tends to reduce the H2:CO ratio of the synthesis gas, however.
For successful commercial scale operation a catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Dietz III and Schmidt (Catalysis Letters (1995) 33:15-29) describe the effects of 1.4-6 atmospheres pressure on methane conversion and product selectivities in the direct oxidation of methane over a Rh-coated foam monolith. The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors. One of the most important of these factors is the choice of catalyst composition. In most of the existing syngas production processes it is difficult to select a catalyst that will be economical for large scale industrial use, yet will provide the desired level of activity and selectivity for CO and H2 and demonstrate long on-stream life. Moreover, such high conversion and selectivity levels must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort in this field continues to be devoted to the development of catalysts allowing commercial performance without coke formation. Also, in order to overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, and to make possible the operation of the reactor at high gas space velocities, various types of structures for supporting the active catalyst in the reaction zone have been proposed. For example, U.S. Pat. No. 4,844,837 (R. M. Heck and P. Flanagan/Engelhard Corporation) describes certain catalysts for the partial oxidation of methane. Those catalysts contain a platinum group metal, optionally supplemented with one or more of chromium, copper, vanadium, cobalt, nickel and iron, and supported on a high surface area alumina-coated refractory metal oxide monolith. The alumina coating is stabilized by a rare earth metal oxide and/or alkaline earth metal oxide against an undesired high temperature phase transition to alpha alumina.
A number of process regimes have been proposed for the production of syngas via partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
A fixed bed reactor configuration would alleviate the catalyst degradation, but would require a pressure differential (pressure drop) to allow gas flow over the catalyst. To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process comprise ruthenium, rhodium, palladium, osmium, iridium, and platinum. U.S. Pat. No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr−1 to 12,000,000 hr.−1 The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
Catalysts containing Group VIII metals such as nickel on a variety of supports are known in the art. For example, V. R. Choudhary et al. (J. Catal. (1997) 172:281-293) disclose the oxidative conversion of methane to syngas over certain nickel-based catalysts supported on low surface area porous carriers precoated with alkaline and rare earth oxides.
U.S. Pat. No. 5,149,464 describes a method for selectively converting methane to syngas at 650° C. to 950° C. by contacting the methane/oxygen mixture with a solid catalyst, which is either:
a catalyst of the formula MxM′yOz where:
M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr and Hf;
Ln is at least one member of lanthanum and the lanthanide series of elements;
M′ is a d-block transition metal, and each of the ratios x/z and y/z and (x+y)/z is independently from 0.1 to 8; or
an oxide of a d-block transition metal; or
a d-block transition metal on a refractory support; or
a catalyst formed by heating a) or b) under the conditions of the reaction or under non-oxidizing conditions. The d-block transition metals are stated to be selected from those having atomic number 21 to 29, 40 to 47 and 72 to 79, the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafiium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold. Preferably M′ is selected from Fe, Os, Co, Rh, Ir, Pd, Pt and particularly Ni and Ru. The exemplary conversions, selectivities, and gas hourly space velocities are relatively low however, while reaction temperatures are relatively high, and the effects of coke formation are not addressed.
U.S. Pat. No. 5,855,815 (Park) describes certain alkali-element promoted NiO catalysts supported on silicon-containing supports. These catalysts are used for CO2 reforming of methane with or without the addition of O2 and H2O.
E. Ruckenstein et al. (Applied Catalysis A: General 183:85-92 (1999)) describe certain solid solution catalysts for partial oxidation of methane and CO2 reforming of methane containing NiO supported on MgO, SiO2, AlO3 or La2O3. Solid solution systems are formed by metal oxides that have comparable physical and chemical properties, such as crystallographic structures, lattice dimensions and bond distances. In addition, the electronegativity of the metals sharing the oxygen determines the stability of the resulting catalyst. As stated by Ruckenstein et al. (Ind. Eng. Chem. Res., 1998, 37, 1744-1747):                “NiO and MgO are both face-centered-cubic oxides with very close lattice parameters (4.1496 and 4.2112 Å for NiO and MgO, respectively) and bond distances (2.10 and 2.11 Å for NiO and MgO, respectively). With the electronegativity of Mg (1.293) being lower than that of Ni (1.8), the binding of oxygen is stronger to Mg than to Ni, it is difficult to reduce NiO to Ni(0) metal, thus improving the coking resistance of the Ni catalyst.”        
U.S. Pat. No. 5,744,419 (Choudhary et al.) describes certain supported Ni and Co catalysts, with noble metal promoters, that are employed for the production of syngas by partial oxidation with oxygen or oxidative steam and/or CO2 reforming with oxygen of methane or light hydrocarbons.
U.S. Pat. No. 5,368,835 (Choudhary et al.) and U.S. Pat. No. 5,338,488 (Choudhary et al.) describe certain Ni-based composite catalysts containing various rare earth or alkaline earth elements, for catalyzing the production of synthesis gas by oxidative conversion of methane.
V. R. Choudhary et al. (J. Catalysis 178:576-585 (1998)) describe processes for the oxidative conversion of methane to syngas catalyzed by NiO supported on various oxides such as CaO, MgO and rare earth oxides. Support effects on NiO in the partial oxidation of methane to syngas are discussed. The beneficial effects of adding Co to certain NiO catalysts for oxidative conversion of methane to syngas have also been described (Choudhary et al., Applied Catalysis A: General 162:235-238 (1997)). The selective oxidation of methane to CO and H2 over certain Ni/MgO and NiO/MgO catalysts at low temperature are described by V. R. Choudhary et al. in Angew. Chem. Int. Ed. Engl. (1992) 31:1189-1190. Choudhary and Mamman also describe methane to CO and H2 conversion reactions involving partial oxidation by O2, oxy-steam reforming, oxy-CO2 reforming, CO2 reforming, and simultaneous steam and CO2 reforming over certain NiO—MgO solid solution catalysts (Applied Energy (2000) 66:161-175).
A potential disadvantage of many of the existing catalytic hydrocarbon conversion methods is the need to include steam in the feed mixture to suppress coke formation on the catalyst. Typically, the ratio of steam to methane, or other light hydrocarbon, in the feed gas must be maintained at 1:1 or greater. The volume of gaseous H2O significantly reduces the available reactor space for the production of synthesis gas. Another disadvantage of using steam in the production of syngas is that steam increases the production of CO2, which is carbon that is lost to the process of making CO product. Other existing methods have the potential drawback of requiring the input of a CO2 stream in order to enhance the yield and selectivity of CO and H2 products. Another drawback of some existing processes is that the catalysts that are employed often result in the production of significant quantities of carbon dioxide, steam, and C2+ hydrocarbons. This often renders the product gas mixture unsuitable, for example, for feeding directly into a Fischer-Tropsch type catalytic system for further processing into higher hydrocarbon products. Moreover, for efficient syngas production, the use of elevated operation pressures is necessary in order to ensure the direct transition to a downstream process, such as a Fischer-Tropsch process, without the need for intermediate compression.
At the present time, none of the known processes appear capable of sufficiently high space-time yields. Typically, partial oxidation reactor operation under pressure is problematic because of shifts in equilibrium, undesirable secondary reactions, coking and catalyst instability. Another problem frequently encountered is loss of noble metals due to catalyst instability at higher operating temperatures. Although advancement has been made toward providing higher levels of conversion of reactant gases and better selectivities for CO and H2 reaction products, problems still remain with finding sufficiently stable and long-lived catalysts capable of conversion rates that are attractive for large scale industrial use. Accordingly, a continuing need exists for better processes and catalysts for the production of synthesis gas, particularly from methane or methane containing feeds. In such improved processes the catalysts would be stable at high temperatures and resist coking. They would also retain a high level of conversion activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity and elevated pressures for long periods of time on-stream.